This article argues that niche party formation is only one of several substitutable strategies for niche activists seeking policy influence. Other organizational mechanisms are argued to be superior to political parties under certain institutional conditions. I introduce a formal model of how activists achieve policy influence by choosing to support the formation of whichever type of organization will optimally send a signal of electoral threat to mainstream politicians. Institutions determine the different access costs for the various organizational mechanisms that can be used to send this signal, and also determine the associated response costs to mainstream politicians of making concessions to different organizational mechanisms. Therefore under particular institutional circumstances that affect these two cost parameters, interest group entry may surpass party entry as the optimal strategy for activists. The predictions of the model are assessed via a nested logit model using cross-national survey data on how activists have allocated their individual support to different organizational mechanisms.